Author
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Norvell, Wendell |
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ADAMS, M.L. - CORNELL UNIVERSITY |
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Submitted to: Journal of Plant Nutrition
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 1/5/2006 Publication Date: N/A Citation: N/A Interpretive Summary: Many important food crops, including soybeans, are susceptible to iron deficiency when grown on alkaline soils that are high in soluble bicarbonate. Iron deficiency causes reduced growth and productivity, and is visually evident as "chlorosis", a yellowing of leaves caused by decreased chlorophyll. Efforts to improve susceptible crops by plant breeding have been aided by simple and reproducible screening methods based on short-term growth in hydroponic culture solutions. Although successful for some crops, methods using sodium or potassium bicarbonate may result in undesirably high levels of monovalent cations, which are atypical of most soil solutions, and which may lead to specific sodium toxicities or excess potassium accumulation in susceptible plants. A modified screening method was developed using carbon dioxide-enriched aeration of magnesium bicarbonate solutions to create a more soil-like solution composition and to replace sodium or potassium salts. Three varieties of soybeans ranging in chlorosis-susceptibility from susceptible to resistant were grown and compared in screening solutions containing either sodium or magnesium bicarbonate. Results for the two screening solutions were very similar, providing identical chlorosis-susceptibility rankings for the three cultivars. Measured values in leaves of chlorophyll, iron, and visual chlorosis index were closely related, providing equivalent measures of chlorosis resistance. Substitution of magnesium bicarbonate for sodium or potassium bicarbonate provided a more soil-like composition for chlorosis-screening solutions and avoided potential problems from excessive uptake of sodium or potassium. Technical Abstract: Many food crops, including soybeans, are susceptible to iron deficiency when grown on alkaline soils that are high in soluble bicarbonate. Efforts to increase the resistance of crops to iron deficiency chlorosis by plant breeding has been aided by simple and reproducible screening methods based on short-term growth in hydroponic culture solutions. Current screening methods use NaHCO3 or KHCO3 as sources of bicarbonate-alkalinity in the culture solutions. However, the high concentrations of Na or K in these solutions are not typical of most alkaline soil solutions, and they may lead to specific sodium toxicities in sensitive plants or excessive potassium accumulation. Using MgO as the source of alkalinity in combination with CO2-enriched aeration produced a screening solution that was more typical of alkaline soil solutions found in many soybean-producing soils. This modified screening solution was tested against the NaHCO3-CO2 method of Chaney et al. (1992) using three soybean varieties of known resistance to Fe-deficiency chlorosis (A7, Williams, Wayne). Bicarbonate alkalinity was 10 mM with aeration by 3% CO2, and Fe was provided as FeDTPA (diethylenetriaminepentaacetic acid) at 15 uM (low Fe) or 60 uM (adequate Fe). Four plants of each cultivar were grown for 23 days. Leaf chlorophyll, Fe concentration, and visual chlorosis index were measured on trifoliolate leaves 1, 2, and 3. Leaf chlorophyll, visual chlorosis index, and leaf Fe concentration were closely related (R^2 = 0.76 to 0.87), providing equivalent measures of chlorosis resistance. Results for the two screening solutions were very similar, providing identical chlorosis-susceptibility rankings for the three cultivars. Substitution of Mg(HCO3)2 for NaHCO3 or KHCO3 provides a more soil-like composition for chlorosis-screening solutions and avoids potential problems from excessive uptake of Na or K. |